Rot is one of the 7 elemental damage types.

Rot damage is usually the third elemental spell a mage learns, often while still on Silan. It is generally the preferred type of damage against intelligent plants until the mage reaches level 125 and can learn the racial damage types, and perhaps even after that.

To increase your chances of resisting a Rot elemental spell, you have to wear jewelry that raises your Prime Roots resistance.

Cutes are capable of casting Rot elemental spells.

Najabs have a spit attack which does Rot damage, though the attack itself is physical and not magical in nature. That means Rot protection will protect you against it, but Prime Roots resistance will not help you resist it.